Advanced UNIX progamming
Fall 2002
Instructor: Ashok Srinivasan
Lecture 5
Acknowledgements: The syllabus and power point presentations are modified versions of those by
T. Baker and X. Yuan
Announcements
• Reading assignment
– APUE Chapter 1
– APUE Chapter 3
• Pages 47-56, 56-62
– APUE Chapter 4
• Pages 77-81, 92-95
– APUE Chapter 5
Review
• Review some features of C
– Header files
– Command line arguments
– Utilities
• Review some UNIX system calls
– system, etc
• Portability
– Standards: ANSI, POSIX, etc
– 32 bit vs 64 bit
– Byte order: Little endian vs big endian
Week 2 Topics
• Review some features of C
– Header files
– Command line arguments
– Utilities
• Review some UNIX system calls
– system, etc
• Portability
– Standards: ANSI, POSIX, etc
– 32 bit vs 64 bit
– Byte order: Little endian vs big endian
Week 2 Topics ... continued
• Introduction to the UNIX API
–
–
–
–
Environment variables
Exit status
Process ID
User ID
• UNIX file system
– File system abstraction
– Directories
– File descriptors
Review some features of C
• Header files
• Macros
• Command line arguments
– See example1.c
• Utilities
Some Unix System Calls
• You may use these in your first
assignment
– system
– mkstemp
– See example2.c
system
#include <stdlib.h>
int system(const char *string);
– Works as if string is typed into the shell at a
terminal
– Returns the exit status (see man page for
waitpid)
– Usually -1 is returned if there is an error
mkstemp
#include <stdlib.h>
int mkstemp(char *template)
– template should end in XXXXXX
– It replaces XXXXXX with unique file name,
and returns an open file descriptor for a file
available for reading and writing
Portability
• Standards
– Source code portability: ANSI/ISO C
– UNIX standards: POSIX, open group
– Internet engineering task force (IETF)
• 32 bit vs 64 bit
• Byte order
– Little endian vs big endian
Source Code Portability
• Standard programming language
– Example: ANSI/ISO C
• ISO C90 is in use; C99 is latest - should it be used?
• Standard libraries
• Standard API to operating system
– Example: POSIX.1
• Auto-configuration mechanisms
• Programmer discipline
Unix Standards
• POSIX (IEEE STDS 1003.x and
ISO/IEC 9945)
– POSIX.1: System API for C language
– POSIX.2: Shell and utilities
– POSIX.5: System API for Ada language
– POSIX.9: System API for Fortran language
• See also http://www.pasc.org and
http://www.standards.ieee.org
Unix Standards ... continued
• The Open Group
– A consortium of vendors and user organizations
– Consolidation of X/Open and the Open Software
Foundation
– Controls the UNIX trademark
– The Austin Group combined the IEEE, TOG, and
ISO standards
• See also http://www.opengroup.org and
http://www.opengroup.org/onlinepubs/007904
975
IETF
• Internet Engineering Task Force (IETF)
– Network designers, operators, vendors,
researchers
– Deals with the Internet
– Issues RFCs
• See also http://www.ietf.org
64-bit vs. 32-bit architecture
• Pointers cannot be stored as
int
• size_t cannot be stored as
int
• long may not be long
enough for size_t and off_t
Datatype ILP32
LP64
char
8
8
short
16
16
int
32
32
long
32
64
pointer
32
64
(long long) 64
64
Note: ILP32 and LP64 are not the only two models
Byte order
• Little-Endian
– Low-order byte is
stored at lowest
address
• Big-Endian
– High-order byte is
stored at lowest
address
Introduction to the UNIX API
•
•
•
•
Environment variables
Exit status
Process ID
User ID
Environment Variables
• Associated with each process is a set of
environment variables, each having a value
– These values are passed to the process when it is
executed
– Along with argc and argv, this is another way of
passing parameters to a program
– Example: when in the Bourne shell you execute
export DISPLAY=mysystem:0.0
The environment variable DISPLAY has value mysystem:0.0
– A process can access its environment variables
using either (i) external variable environ, or (ii)
function getenv()
• See example3.c
Exit status
• Each process (except the system's initial
process) has a parent process
– When a process terminates, its parent can find out
what caused the termination, via a status value
• The termination status contains a short integer exit status
code
• The function exit() allows a process to terminate itself
and specify the exit status code that it wants to return to
the parent
– When a command is run from a shell, the exit
status code is returned to the shell, which allows it
to be used in job control statements
– What is the difference between _exit() and exit()?
Process ID
• Each process has a unique identifier, of
signed arithmetic type
pid_t
• The process ID of the current process
can be obtained by calling the function
getpid()
User ID
• Associated with each process are two
user ID's
– A fixed real user ID, and
– A changeable effective user ID
• In order to modify privileges
– The real user ID can be obtained by calling
getuid()
– See example4.c
UNIX file system
• File system abstraction
• Directories
• File descriptors
File system abstraction
• File: a sequence of bytes of data
• Filesystem: a space in which files can be
stored
• Link: a named logical connection from a
directory to a file
• Directory: a special kind of file, that can contain
links to other files
• Filename: the name of a link
• Pathname: a chain of one or more filenames,
separated by /'s
File system abstraction ... continued
• inode: a segment of data in a filesystem that
describes a file, including how to find the rest of
the file in the system
• File descriptor: a non-negative integer, with a
per-process mapping to an open file description
• Open file description: an OS internal datastructure, shareable between processes
Directories
Directories ... continued
•
•
•
•
•
Names belong to links, not to files
There may be multiple hard links to one file
Renaming only renames one link to that file
Unix allows both hard and soft links
A file will exit even after the last hard link to it
has been removed, as long as there are
references to it from open file descriptions
– Soft links do not prevent deletion of the file
• A directory may have multiple (hard) links to it
– But this capability is usually restricted, to prevent
creation of directory cycles
File Descriptors
• Each open file is associated with an open file
description
– Each process has a (logical) array of references to
open file descriptions
– Logical indices into this array are file descriptors
• These integer values are used to identify the files for I/O
operations
– The file descriptor 0 is reserved for standard input,
the file descriptor 1 for standard output, and the
file descriptor 2 for the standard error
File Descriptors ... continued
File Descriptors ... continued
• The POSIX standard defines the following
– File descriptor: A per-process, unique,
nonnegative integer used to identify an open file
for the purposes of file access
– Open file description: A record of how a process
or group of processes are currently accessing a
file
• Each file descriptor refers to exactly one open file
description, but an open file description may be referred
to by more than one file descriptor
• A file offset, file status, and file access modes are
attributes of an open file description
– File access modes: Specification of whether the
file can be read and written
File Descriptors ... continued
– File offset: The byte position in the file where the
next I/O operation through that open file
description begins
• Each open file description associated with a regular file,
block special file, or directory has a file offset
• A character special file that does not refer to a terminal
device may have a file offset
• There is no file offset specified for a pipe or FIFO
(described later)
– File status: Includes the following information
• append mode or not
• blocking/nonblocking
• Etc
File Descriptors ... continued
– FIFO special file: A type of file with the property
that data written to such a file is read on a first-infirst-out basis
– Pipe: An object accessed by one of the pair of file
descriptors created by the pipe() function
• Once created, the file descriptors can be used to
manipulate the pipe, and it behaves identically to a FIFO
special file when accessed this way
• It has no name in the file hierarchy
File Descriptors ... continued
• Important points
– A file descriptor does not describe a file
• It is just a number that is ephemerally associated with a
particular open file description
– An open file description describes a past "open"
operation on a file; its does not describe the file
– The description of the file is in the inode
• There may be several different open file descriptors (or
none) referring at it any given time